Bone formation relies on the interaction of various cell types, but it is cells of the osteoblast lineage that are responsible for synthesis, deposition and mineralization of matrix (23). Bone is formed of two types of osseous tissue; compact bone and trabecular bone. Compared to compact bone, trabecular bone has a higher surface area but is less dense, softer, weaker, and less stiff. The primary anatomical and functional unit of trabecular bone is the trabecula. A trabecula is a small, often microscopic, tissue element in the form of a small beam, strut or rod, generally having a mechanical function. On histological section, a trabecula can look like a septum, but in three dimensions they are topologically distinct, with trabeculae being roughly rod or pillar-shaped and septa being sheet-like. Trabecular bone typically occupies the interior region of bones. Trabecular bone is highly vascular and frequently contains red bone marrow where hematopoiesis, which is the production of blood cells, occurs.
The study of bone formation and the factors and mechanisms that regulate this process are often studied in vitro. Studies using primary osteoblast cultures are often used, but immortalized osteoblastic cell lines can also be used. The most extensively studied primary osteoblast cultures are derived from calvaria (skull) or by differentiation of bone marrow-derived mesenchymal stem cells (MSCs).
MSCs from bone marrow have the capacity to differentiate into cells of the connective tissue lineage including bone, fat, cartilage and muscle (23). Calvarial osteoblasts are derived from the differentiation of mesenchymal cells, which have also been reported to be multipotent; it has been demonstrated that they also have the potential to enter the chondrogenic and adipogenic lineages (24). Cultured embryonic and newborn murine calvarial cells are usually a mixed population of MSCs, osteoprogenitors and osteoblasts, and the relative amounts of these different populations depends on the age of the calvaria and the culture conditions (25). However, the signals that regulate the potency, expansion, and commitment of mesenchymal stem cells of the osteoblast lineage remain to be defined.
Understanding the mechanisms that regulate and enhance bone anabolic processes (bone growth) is critical for developing treatments for a number of conditions and diseases of the bone. For example, osteoporosis is a disease of the bone that affects approximately 8 million women and 2 million men in the United States. Osteoporosis is a disease of the bone in which the amount of bone is decreased and the strength of trabecular bone is reduced, cortical bone becomes thin and bones are susceptible to fracture. Osteoporosis is a condition that features loss of the normal density of bone and occurs most often in older people and post-menopausal women (26, 30). Current treatments of osteoporosis include calcium supplementation and different classes of osteoclast inhibitors, such as bisphosphonates. However, these treatments can have unwanted side effects. One of the side effects of bisphosphonates is damage to the esophagus; including esophagitis, esophageal erosions and esophageal ulcers. Other common side effects include inflammation of the eyes, musculoskeletal pains, and jaw necrosis. Common side effects of such drugs are described in detail in Physicians' Desk Reference. 62nd ed. Thomson Healthcare, 2008.
Enhancing bone anabolic functions (bone growth) by increasing the commitment of MSCs to the osteogenic lineage, and thereby increasing osteogenesis represents a promising new therapeutic strategy for osteoporosis and other diseases and conditions of the bone, such as those described supra. However, at present, little is known about the mechanisms and factors that govern MSC commitment to the osteogenic lineage, and even less is known about how to identify these important factors. Therefore, what is critically needed in the art are methods for identifying new agents or factors that can be used to enhance bone anabolic functions and treat conditions and diseases of the bone. The present invention provides such methods.